An average EKG machine
contains: 1) electrode wires with gel patches; 2) a
computer which converts the signal into a waveform and
adjusts for electrical artifact; 3) a screen to view the
cardiac rhythm in real time (many incorporate a 6 second
delay), a printer to produce a hard copy of the waveform,
and many have a method of storing digitially the waveform
data. The data is collected from the patient using a
total of 10 electrodes. Electrodes are patches made of
electro-conductive material, usually an electrolyte gel.
They attach to the patients chest and limbs using a tape
like adhesive. Excessive hair can decrease conductivity
and compromise the data collection. The electrodes
connect to the EKG machine through wires. The procedure
is non-invasive and painless, and the actual data
sampling for a 12 lead EKG usually takes only 5-10
seconds.. Below is a series of diagrams and explanations
of the leads represented by combining specific pairs of
electrodes. Remember: an ELECTRODE is a physical patch
which connects to the patient; a LEAD is a specific
vector in which voltage is measured. In many leads, a central terminal or common ground is
used. This is a computed virtual negative ground
calculated by combining values from several electrodes.
The central
terminal allows the EKG to provide information about
vectors not represented by specific pairs of electrodes.
This will be explained further below.

Lead I is the first of three standard limb leads (I, II,
III). Limb leads measure cardiac depolarization in the
frontal (coronal) plane.

The negative electrode is connected to
the RIGHT ARM.
The positive electrode is connected to the LEFT ARM.

The axis is 0 degrees.

When an action potential
starts on the right and proceeds toward the left side of
the heart, a positive inflection will be seen in lead
one. This holds true for all leads. Whenever a current
proceeds toward a positive electrode, an upright
inflection is seen on the EKG tracing.

Lead II is used alone quite frequently. Normal rhythms
present with a prominant P wave and a tall QRS.

The negative electrode is connected to the RIGHT
ARM.
The positive electrode is connected to the LEFT LEG.

The axis is +60 degrees.

In all the limb leads, the electrodes
may be positioned close to the torso. For convenience,
they are often placed at the shoulders and hips.

Lead III is the last of the three standard limb leads.

The negative electrode is connected to the LEFT
ARM.
The positive electrode is connected to the LEFT LEG.

The axis is 120 degrees.

In addition to the 3 basic limb leads, there are 3
augmented limb leads. These leads provide additional
vector views of cardiac depolarization in the frontal
plane.

Unlike leads I, II, III,
the augmented leads utilize a central negative terminal.
This virtual "electrode" is calculated by the
EKG computer to measure vectors originating roughly at
the center of the heart.

Augmented Lead (RIGHT) avR

The negative electrode is the central
termininal.
The positive electrode is connected to the RIGHT ARM.

The axis is -150 degrees.

Augmented Lead (LEFT) avL

The
negative electrode is the central termininal.
The positive electrode is connected to the LEFT ARM.

The axis is -30 degrees.

Augmented Lead (FEET) avF

The
negative electrode is the central termininal.
The positive electrode is connected to the LEFT LEG.

The axis is +90 degrees.

As discussed above, each limb lead
consists of the difference in electrical potential
between two electrodes. One electrode is designated
negative and one positive.

Leads
I, II, III together form the vectors of an equilateral
triangle (0, 60, 120 degrees). This is called Einthoven's
triangle. Einthoven found that the sum of current of Lead
I + Lead III = Lead II.

He also found that since the heart was
approximately centered in the triangle, the vectors could
all be shifted inward. Taking the edges of the trangle
and drawing them together until their centers intersect
results in a triaxial reference figure. This image can be
seen in the vectors webpage.

Super-imposing the augmented leads over
Einthoven's triangle demonstrates the vector
relationship. Using a common terminal the augmented leads
provide vector views not offered by the limb leads. They
AUGMENT the coronal view of the heart with additional
measurements. Their vectors are: -150, -30, +90 degrees.
See the vector page for comprehensive diagrams of the
limb leads.

Precordial leads make up the other 6 leads and electrodes
connected during an EKG. These measure depolarization in
the axial plane using a central terminal. These leads
provide essental information used in localizing ischemic
or infarcted tissue during a cardiac event.